= Dispersive atom-cavity coupling
<Dispersive atom-cavity coupling> is the off-resonant regime where real excitation exchange is suppressed but virtual mixing shifts the energies. For a <two-level system> describing an <chemical atom> coupled to one mode, the scale $\chi\simeq g^2/\Delta$ appears when $|\Delta|$ is large compared with the relevant occupation-enhanced coupling $g\sqrt{n+1}$.
An effective term proportional to $\hbar\chi a^\dagger a\sigma_z$ then makes the atomic phase depend on <photon> number and the cavity frequency depend on atomic state. Measuring that phase can infer field occupation without resonantly absorbing a <photon> in the ideal approximation. Finite detuning, losses and probe backaction still matter; dispersive does not mean completely uncoupled or universally disturbance-free. A model must state detuning sign and the retained perturbative order.
Back to article page
